The current invention is directed towards a method for providing a modified lens shape for an uncut lens blank, in particular for reducing the thickness of the uncut lens shape. Further, the current invention is directed towards an uncut lens blank for manufacturing a spectacle lens.
In recent years, the number of so-called individually designed spectacle lenses has been significantly increasing. These individually designed spectacle lenses take into account a number of individual parameters of an intended wearer. Further, the demand for a correction not only of lower-order aberrations but also for higher-order aberrations is steadily increasing. This all leads towards more and more lenses being designed with “free form surfaces”, that is, surfaces that do not inhibit any symmetry any more. A full surface profile is determined, for example by providing sagitta for each surface over the whole area, and forwarded to a manufacturing site, for example for grinding, polishing, coating and/or edging. Commercially available wavefront sensors provide the ability and data processing techniques for correcting the high-order aberrations of the eye with an ophthalmic lens without introducing more significant low-order wavefront aberrations; and to the ability of machining ophthalmic lens surfaces of suitable complexity, in particular with modern free-form surfacing techniques.
Prior to edging, that is, cutting the lens blank to its final shape to fit into a frame, the lenses are processed as so-called “uncut lens blanks”, that is, lens blanks having a circular or elliptical shape and a front surface having a convex curvature providing for a specific focal power already applied to it. Further, the front surface may already be coated. The intended optical properties are then provided by surfacing the back surface with a predetermined free-form surface. At least for the surfacing steps, that is, grinding and polishing, and for the step coating, the uncut lens blank has to be blocked by corresponding block pieces that can grip the uncut lens blanks securely while leaving open the surfaces to be processed. Further, the tools and methods used for surfacing inherit certain constraints on the shape of surfaces that can actually be processed. Last, wearers prefer light-weight spectacles and certain aesthetically advantageous frame shapes can fix lenses up to a certain outer edge thickness only.
In the prior art, the problems associated with large lens thicknesses have already been known, in particular for lenses having a very high positive or negative focal power. For these purposes, lenticular lenses have been provided which comprise a lenticular portion providing for desired optical properties, in particular according to a prescription, and a relatively thin margin portion outside of a specific viewing angle for manufacturing and framing purposes. Examples for such lenticular lenses are provided, for example, in “System for ophthalmic dispensing” by Clifford W. Brooks and Irvin M. Borish, ISBN-13: 978-0-7506-7480-5, Third Edition, Butterworth-Heineman/Elsevier, 2007, in particular pages 425 to 429.
Turning to free-form surfaces and corresponding spectacle lenses, the basic problem is still to modify the thickness profile of an ophthalmic lens by modifying one of the surfaces outside of some specified boundary. The traditional and “ideal” method in terms of maximizing the size of the optical zone while minimizing the center thickness of plus lenses or edge thickness of minus lenses, and also minimizing wearer discomfort when the boundary between the lenticular portion and the margin portion lies within the frame, requires that the modified surface has a slope discontinuity along the boundary.
The traditional methods are not well suited to the current back surface freeform technology. The positive powered lenses require specialized pucks with complex front surfaces, which would increase the complexity and the required inventory for freeform processing. The minus lenses require the back to be surfaced and polished twice, adding to the processing time and cost. In addition methods and surfaces are perceived as “higher tech” and are aesthetically preferred.
WO 2014/060552 A1 shows a method for determining a surface of an ophthalmic lens comprising a carrier and a Fresnel membrane lying on the carrier. The invention also relates to an ophthalmic lens comprising such a surface. The invention especially relates to a method for determining a surface of an ophthalmic lens, where the surface comprises a Fresnel membrane and a carrier supporting the Fresnel membrane. The carrier has a geometric center, a first central region and an annular peripheral region, and a rotational symmetry. The Fresnel membrane, the first central region and the peripheral region are centered on the geometric center, the first central region being defined by a first circular border, and the peripheral region being defined by a second circular border and by the edge of the surface. The method comprises steps of: (SI) determining a first curvature profile of the carrier in the first central region and a second curvature profile of the carrier in the peripheral region; (S10) determining a first radius of the first border and a second radius of the second border; (S20) determining a third curvature profile of a transition region of the carrier, where the transition region is adjacent to the first central region and to the peripheral region; (S30) determining a target curvature profile of the surface where the target curvature profile is identical in the first central region to the first curvature profile of the carrier; (S40) determining a continuous curvature profile for the Fresnel membrane from a difference between the target curvature profile and the curvature profile of the carrier; and (S50) determining the Fresnel membrane by cutting the continuous curvature profile for the Fresnel membrane.
Hence, it is an object of the current invention to provide for a method for reducing the thickness of an uncut lens blank, in particular having a spherical front surface and an arbitrary free-form back surface shape, and a corresponding uncut lens blank.